gdm-plymouth ?
Isn’t that from the AUR?
display-manager.service should symlink to gdm.service. I’m not sure where gdm-plymouth.service came from, nor if you intentionally installed it from the AUR.
Straight from the PKGBUILD page, an important warning:
I would uninstall gdm-plymouth and force the reinstallation of gdm, then see if that resolves it.
That should do the trick.
However, If uninstalling gdm-plymouth and reinstalling gdm isn’t sufficient, then continue to proceed to “enable” gdm.service once again.
EDIT: If you absolutely need/want gdm-plymouth, then you need to manually break the current symlink to gdm-plymouth.service, and then (re-)create the proper symlink to gdm.service.
display-manager.service → gdm.service